Abstract: An automatic shut-off device for a fluid carrying pipe includes a flow sensor switch housing which is connectable in line with the pipe. A sensor switch is included in the housing, which switch is operated by a switch paddle positioned within the housing in the path of fluid flow there through. A venturi is inserted with an inlet diameter positioned proximate an inlet end of the housing and the venturi has an outlet with a smaller outlet diameter positioned proximate the switch paddle. The venturi both speeds up the flow of liquid through the housing and directs the flow onto the paddle, both of which increase sensitivity of the switch. The venturi is designed to be either left intact or cut off when installed to allow the sensitivity of the flow sensor switch to be adjusted upon installation.